You need to know at least one spec so you can measure the conditions at which that occurs then you can calculate the VA rating. If you know what the loaded down secondary voltage should be, load the transformer secondary down while monitoring the secondary current and voltage (make sure the primary voltage is close to what it should be or your numbers will contain some error). When the voltage drops to the spec as you increase the load on the secondary check to see how much current is being drawn and then multiply the two numbers. If there are two secondaries double that number.
Alternatively, you can get a rough estimate by using the old VA/lb trick. I don't know the numbers off hand but you can probably find some good ones if you search.
Alternatively, you can get a rough estimate by using the old VA/lb trick. I don't know the numbers off hand but you can probably find some good ones if you search.
If you are talking toroids, rough ballpark figures;
<500VA => 2.25lbs/100VA
- 500VA => 2.00lbs/100VA
>500VA => 1.75lbs/100VA
Transformers from a couple of manufacturers have higher numbers, some lower.
EI cores are too far spread for estimates.
An accurate way is to calculate the efficiency by measuring the ingoing and outgoing VA numbers.
A proper transformer will have the highest efficiency at the VA rating it is designed for.
I assume you know the design secondary voltage or the way of determining it.
<500VA => 2.25lbs/100VA
- 500VA => 2.00lbs/100VA
>500VA => 1.75lbs/100VA
Transformers from a couple of manufacturers have higher numbers, some lower.
EI cores are too far spread for estimates.
An accurate way is to calculate the efficiency by measuring the ingoing and outgoing VA numbers.
A proper transformer will have the highest efficiency at the VA rating it is designed for.
I assume you know the design secondary voltage or the way of determining it.
A more scientific way to determine VA capability is to power up the transformer on your bench and measure the no-load secondary voltage. Now add some resistance (big power resistors) across the secondary until the voltage drops by 30% for small transformers (<50 VA), 20% for medium sized transformers (50 to 100 VA) and 12-15% for big suckers (>>100VA). For high efficiency semi-toroidal transformers, all voltage drops will be roughly 1/2 of these listed. Make note of the voltage under load and use ohms law to discover VA rating:
* Rated secondary current = E/I = voltage under load/resistance in ohms
* VA rating in watts = I*R = current from above formula * voltage under load
* Rated secondary current = E/I = voltage under load/resistance in ohms
* VA rating in watts = I*R = current from above formula * voltage under load
burnedfingers said:
Can you show us a picture?
I have loaded a transformer and approximated the VA buy means of percentage in voltage drop, just as dong says in the above post. You can look at the tables on the plitron web site that show % regulation by VA. Togther this info would get you very close if not exact
Gee whiz I like the peranders method, now I need one of those infrared heat sensor tools.
Shawn.
The first method, the experts tells burnedfingers must be the easiest for burnedfingers 
This is way i do it.
1- measure output ac voltage.
2-Bridge rectify and filter with like 4700Uf capacitor.Now dc voltage will be 1.4Vac.
3-Connect variable load for example bulbs in series. See when Vac not loaded =VDc loaded.
4-Measure the curent through load
5-Calculate output power Vdc X Adc.
This will be ouput power of that winding.If transformer has more windings,repeat test for each winding then add them all to find
total power.
Basis of this procedure is that nomally transformers are made to provide voltages at rated curent. For example a 12v 4A transformer will provide arround 16V when reeied and filtered
when 4A load will be connected it will drop to 12Vdc.
This procedure i figured out myself anit worked fine for me.
1- measure output ac voltage.
2-Bridge rectify and filter with like 4700Uf capacitor.Now dc voltage will be 1.4Vac.
3-Connect variable load for example bulbs in series. See when Vac not loaded =VDc loaded.
4-Measure the curent through load
5-Calculate output power Vdc X Adc.
This will be ouput power of that winding.If transformer has more windings,repeat test for each winding then add them all to find
total power.
Basis of this procedure is that nomally transformers are made to provide voltages at rated curent. For example a 12v 4A transformer will provide arround 16V when reeied and filtered
when 4A load will be connected it will drop to 12Vdc.
This procedure i figured out myself anit worked fine for me.
Re: Re: How to measure transformer VA?
If you really have to do this without expensive measurement tools, you can immerse the transformer in vegetable oil, measure the temperature as you increase the load, allowing time for the temperature to settle as the load is increased. As the core becomes less efficient the temperature rise should accelerate.
peranders said:
You could also load it and when the temperature is 105 deg C (221 F) in side, then you'll have the rating. 105 deg is pretty
If you really have to do this without expensive measurement tools, you can immerse the transformer in vegetable oil, measure the temperature as you increase the load, allowing time for the temperature to settle as the load is increased. As the core becomes less efficient the temperature rise should accelerate.
Re: Re: Re: How to measure transformer VA?
EWWw i am not going to be soaking my transformers in veggie oil! how the heck would you clean that mess off the transformer when you were done? I dont want my amplifier to smell like French Fries everything i turn it on.
I need to call Amveco again, those guys were great. there has to be a method for accuratly measuring VA. I do believe it was something about deg of temp rise over time with a load.
My shop is rather cold right now. i could use a few 1kva trafos running at 105c in there right about now to warm things up!
Zc
jackinnj said:
EWWw i am not going to be soaking my transformers in veggie oil! how the heck would you clean that mess off the transformer when you were done? I dont want my amplifier to smell like French Fries everything i turn it on.
I need to call Amveco again, those guys were great. there has to be a method for accuratly measuring VA. I do believe it was something about deg of temp rise over time with a load.
My shop is rather cold right now. i could use a few 1kva trafos running at 105c in there right about now to warm things up!
Zc
Re: Re: Re: Re: How to measure transformer VA?
Well, duuuhhhhh, it is Minnesoooooootaaaaaaaaah. Yabetcha!
According to the great bald leader of NPR, you folks only get happy when the temperature is below zero and everyone has the flu.
Zero Cool said:
My shop is rather cold right now. i could use a few 1kva trafos running at 105c in there right about now to warm things up!
Zc
Well, duuuhhhhh, it is Minnesoooooootaaaaaaaaah. Yabetcha!
According to the great bald leader of NPR, you folks only get happy when the temperature is below zero and everyone has the flu.
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